FIREBIRD 90 INSTALLATION?

Just installed firebird 90 which I got from a friend which was just serviced before we removed it from his house.I removed my own warmflow which sprang a leak in the boiler.the pump(the one attached to the water supply) on the old one was faulty so I had a new one to fit.The arrow on the old pump was facing towards the boiler but the new one was facing the other way so I turned it upside down to face it the same way the old came out.I started the burner up and she fired but after it heated up she cut out.have bled all the system.the rads did not heat up,so I checked the exspansion tank and it was filling up and over flowing into main tank.the lower pipe in the tank was warm so I presume the pump was sending the water to tank and not the rads.I removed the pump and turned it around the other way and this stopped the overflow but still no rads heated up.is the pump in the right direction or do I have a air lock some where which caused the overflow?